1. Field of the Invention
The present invention relates to high-quality digital printing in which objects are intermixedly printed, and more particularly, in which a printing process exhibits characteristic, statistically predictable defects which degrade quality. In order to apply a system-specific and defect-specific function to the digital signal to pre-compensate for a defect and thereby increase quality, an efficient search method is used to identify candidate printer defects and apply the correct pre-compensation function.
2. Description of the Related Art
Digital color printers form a digital image for each of several separations, such as cyan, magenta, yellow, and black. The digital image instructs the printing mechanism of the printer in the amount of each color ink to deposit and the method of deposition at each addressable point on the page.
A digitally imaged page can consist of graphical objects such as text, lines, fills, pictures, etc, all imaged in ways which can be isolated from each other, can abut one another at one or more points, can partially overlap one another, or can completely overlap one another. The resulting printed page or graphic image is therefore made up of a patchwork of shapes representing the graphic objects, some of which are "clipped" by objects imaged later in the succession.
In practice, every color printing system has characteristic defects which can cause subtle problems that detract from achieving the highest possible quality color printing. For example, ink jet printing must handle excessive ink coverage which can cause bleeding or spreading of colors and paper distortion. Xerographic printing contends with a different set of problems which can detract from print quality. Examples are "haloing", in which toner in one separation interferes with toner transfer at the same location in another separation, "tenting", which is toner deletion caused by high toner pile casting a mechanical or electrostatic "shadow" which prevents correct development of abutting toner, trail-edge deletion and starvation, which cause toner deletion at certain edges, or misregistration between two colors. Many of these characteristic problems in printing systems can be traced to undesirable interactions between abutting colors on the page.
Despite known problems, the digital image sent to the printer has in the past assumed a perfect printing mechanism, and provided an ideal image to print. While increasingly sophisticated controls have been added to printing mechanisms to reduce defects and come closer to the perfect printer expected by the digital image, electromechanical defects in any printing system are still common and are to be expected at both the low end where system cost restraints preclude use of expensive controls and the high-end where production speeds challenge existing control systems.
Recent work has begun to look at modifying a digital image in advance in order to pre-compensate for expected problems in a printer. A number of approaches to digital data pre-compensation are described in greater detail in the co-pending patent application "Anamorphic Object Optimized Function Application for Printer Defect PreCompensation", which is incorporated herein by reference. The invention described in "Anamorphic Object Optimized Function Application for Printer Defect PreCompensation" is able to detect and correct a wide range of printer problems related to objects of different types being printed adjacent to one another. It uses color and object information such as object type, object size in the scan and process directions, rendering intent, and other relevant object parameters to detect and pre-compensate for a large number of potential printing problems such as trail-edge deletion and starvation (where toner concentrations drop at certain color edges), misregistration, haloing (where toner concentrations drop when certain colors are surrounded by other colors), halftone mismatch (where objects rendered with different halftones meet), and so on.
Because such a large number of printing problems are being addressed, it is important in the detection phase to have a search mechanism which can quickly and efficiently recognize patterns of pixels that are predicted to cause printing problems, and efficiently apply the appropriate pre-compensations to correct those problems.
The co-pending application "Anamorphic Object Optimized Function Application for Printer Defect Pre-Compensation" describes the step whereby a band of scanlines is searched for potential printing defects, but is not particularly concerned with developing an efficient search mechanism. A general method is described whereby printer defects are detected and corrected in a serial, in-line fashion, one module at a time, which achieves the goal of the search step without necessarily achieving optimum efficiency. In the method described, the invention includes a module called a "filter" for each printer defect that is to be identified and corrected. It specifies that each filter for each printing defect must accomplish two tasks: first, it must recognize the conditions in a band of scanlines that will trigger its specific printing defect, and second, it must apply a function that changes the attributes of the affected runs in order to pre-compensate for the expected defect.
To accomplish these detect-and-correct steps, a pointer to the band of scanlines is passed to each of the filters in turn. Each filter searches the band of scanlines in order to determine if the criteria for its particular printing defect have been met. If so, each filter applies a correction to the digital data within that band of scanlines, and control passes to the next filter in the chain which repeats the same steps of analyzing the band of scanlines for defects which it is able to predict, and correcting the defects if found.
The method of allowing each filter in turn to examine the current band of scanlines and correct them is a general search method that will work in all cases and is therefore a preferred embodiment for the general solution described in "Anamorphic Object Optimized Function Application for Printer Defect Pre-Compensation".
The current invention instead proposes a method for detection and correction of printer defects that need not work for the general case of all printer defects but works with most defects and is designed for efficiency. This is important because if many printer defects are to be searched, it is important that system speed not be unduly sacrificed for higher quality.
The current invention proposes that rather than each filter in turn examining the current band of scanlines for printer defects, instead each filter should register with a search module the pattern or patterns of runs that are likely to trigger its particular defect. The patterns can be encoded to be keys to a lookup table of any sort. Then, when a band of scanlines is being analyzed for likely printer defects, the search module encodes the current pattern of runs and uses that encoding as a key to lookup whether any filter has registered the current pattern as likely to cause a problem. If so, control may pass immediately to that filter or module for correction of the digital data.